Coating films of vinylidene chloride copolymers are characterized by excellent barrier properties. Accordingly, films coated with the vinylidene chloride copolymers have been used for making it possible to store food packaged, with oxygen and water vapor cut off, for a long period of time without making the food therein moisturized or oxidized. Of films, some are excellent in barrier properties against oxygen or water vapor. However, films which can cut both oxygen and water vapor off at a high level are rare except for the vinylidene chloride copolymer films. The films coated with the vinylidene chloride copolymers have been widely used mainly as food packaging materials.
Methods for preparing the vinylidene chloride copolymer films are roughly divided into two methods. One is a method of preparing the films by melt extrusion (melt extrusion method), and the other is a method of coating films with aqueous dispersions of the vinylidene chloride copolymers or solutions of the vinylidene chloride copolymers in organic solvents (coating method).
In the case of the melt extrusion method, the polymers are orientated by drawing the vinylidene chloride copolymer films after extrusion, and the films excellent in barrier properties can be prepared. However, when polymers containing 88% or more of vinylidene chloride are intended to be extruded, thermal degradation takes place upon extrusion. Extrusion thereof is therefore practically difficult. In contrast, according to the coating method, even the polymers having a vinylidene chloride content as high as 88% or more can be formed into films, so that films excellent in barrier properties per unit thickness can be obtained. In particular, when solutions of vinylidene chloride copolymers dissolved in organic solvents are applied, coating films very thin in thickness and excellent in barrier properties are obtained because the solutions little contain additives impeding the barrier properties such as plasticizers and surfactants. As the method for preparing a coating film excellent in barrier properties by applying a solution of a vinylidene chloride copolymer dissolved in an organic solvent, for example, JP-B-63-24016 (the term "JP-B" as used herein means an "examined Japanese patent publication") and JP-A-3-54244 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") disclose methods for preparing films excellent in barrier properties by dissolving vinylidene chloride polymers in specific organic solvents.
However, with recent high-level requirements for barrier properties in the market, films more excellent in barrier properties than the conventional films have now been demanded.
It is an object of the present invention to provide a film or a laminated film of a vinylidene chloride polymer more excellent in barrier properties than the conventional ones.